1. Field of the Invention
The present invention relates to an image forming method used in an image display apparatus for displaying data from a computer, a facsimile system or the like or in an electronic copying machine for exposing an original image and forming a copy image.
2. Description of the Prior Art
CRT (cathode-ray tube) type image display apparatuses for displaying data from computers, facsimile systems or the like have become widespread.
The arrangement of another conventional display apparatus using an endless belt, is illustrated in FIG. 1. Referring to FIG. 1, an endless belt photosensitive body (to be referred to as a photosensitive body hereinafter) 8 as an image carrier is looped around guide rolls 9, 10, 11 and 12 in a housing 1 and is intermittently driven by a driving means (not shown). An output laser beam from a semiconductor laser (not shown) modulated in response to an electrical image signal is scanned by a scanner 5 along one direction. Furthermore, the lower surface of the photosensitive body 8 is exposed through an f-.theta. lens 6 and a mirror 7. The photosensitive body 8 comprises a transparent conductive base and a photoconductive layer formed thereon.
A developing unit 15 is arranged to oppose the surface of an exposure position A of the photosensitive body 8. The developing unit 15 has a sleeve 17, and the sleeve 17 has a magnet 16 rotated in a direction indicated by an arrow in FIG. 1. Toner 18 as a developing agent with conductive and magnetic properties is supplied to the surface of the sleeve 17 and is adjusted by a blade 19 to a uniform thickness. The toner 18 is then applied to the surface of the photosensitive body 8. A DC voltage from a DC voltage source (not shown) is applied between the sleeve 17 of the developing unit 15 and the base of the photosensitive body 8. Rollers 13 and 14 are arranged in the vicinity of an exposure and development portion. The photosensitive body 8 is kept flat to keep a constant distance between the surface of the photosensitive body 8 and the sleeve 17 of the developing unit 15. A toner image is written on the surface of the photosensitive body 8 upon irradiation of the beam at the position A opposing the developing unit 15 and is transferred to a display section 2.
The display section 2 has a rectangular window hole 3 in the front surface of the housing 1. The toner image on the photosensitive body can be visually checked through a transparent member 4 covering the window hole 3. When a visible or toner image area coincides with the position of the window hole 3, the photosensitive body 8 is automatically or manually stopped for a desired period of time. The operator can visually check the toner image on the surface of the photosensitive body through the transparent member 4 mounted in the window hole 3.
A lamp 20 is optionally arranged to delete the history of the photosensitive body, if required. The lamp 20 is turned on while the photosensitive body is driven. However, the lamp 20 is turned off when the photosensitive body is stopped.
FIGS. 2 and 3 show an electrophotographic method disclosed in Japanese Patent Laid-Open Nos. 98746/1983 to 98749/1983 (corresponding to U.S. Ser. No. 445,070). The principle of image formation by the photosensitive body 8 in the apparatus shown in FIG. 1 will be described with reference to FIGS. 2 and 3.
FIG. 2 shows a charge state in a bright portion of information light. When the toner 18 applied with a voltage through the sleeve 17 is brought into contact with the photosensitive drum, an electric field is applied to a photoconductive layer 8c. In this case, when information light LB irradiates the photoconductive layer 8c, photocarriers e are generated in the photoconductive layer 8c. The photocarriers are guided by the electric field to the vicinity of the surface of the photoconductive layer 8c. As a result, a strong electrostatic attraction force acts between the toner 18 and the photoconductive layer 8c. The toner 18 is attracted to the photoconductive layer 8c, i.e., the surface of the photosensitive body 8. The sleeve 17 may be rotated while the magnet 16 is fixed.
In this example, the photoconductive layer 8c comprises an n-type semiconductor, and a positive voltage is applied to the toner 18. The carriers e generated from the photoconductive layer 8c in the vicinity of the substrate upon irradiation of the information light LB can be effectively guided toward the surface of the photoconductive layer 8c. A strong electrostatic attraction force acts between the toner 18 and the photosensitive body 8, so that the toner 18 is attached to the photosensitive body.
FIG. 3 shows a charge state of a dark portion of the information light. When an electric field is applied between the toner 18 and a transparent conductive layer 8b of the base, an electrostatic attraction force acts therebetween. However, since the photoconductive layer 8c is present between the toner 18 and the transparent conductive layer 8b and the toner 18 is spaced apart from the conductive layer 8b, the attraction force is small. The toner 18 is removed from the surface of the photoconductive layer, i.e., the surface of the photosensitive body 8 by a magnetic force of the rotary magnet 16 arranged in the sleeve 16 or by an attraction force acting between the toner particles.
In order to change the toner image on the photosensitive body 8, the photosensitive body 8 passes by the exposure position again to form a new toner image.
When the toner carrying portion of the photosensitive body becomes a toner noncarrying portion, the toner 18 having a reduced electrostatic attraction force is removed by the magnetic field of the magnet 16, thus obtaining the toner noncarrying bright portion. However, when the toner carrying portion of the photosensitive body remains stationary, the carriers e are again injected by information light. The toner 18 is held by the information light since the charge of the toner becomes larger than the magnetic field of the magnet 16. Therefore, the toner image on the surface of the photosensitive drum will not adversely affect the next image formation cycle. Any special cleaning means need not be provided.
Referring to FIGS. 2 and 3, a transparent conductive base 8a comprises a polyethyleneterephthalate film for supporting the conductive layer 8b. A voltage from a power source E is applied to the sleeve. When a normal photosensitive body 8 is used, a bias voltage of 100 to 500 V is applied to the body.
In the photosensitive body 8 having the construction described above, a nonimaging portion is predetermined at a seam of the photosensitive body 8 or before the photosensitive body 8 is driven at a constant speed. In the apparatus shown in FIG. 1, reference symbols 8A and 8B denote first and second imaging areas. Areas 8C and 8D between the first and second imaging areas 8A and 8B serve as nonimaging areas, respectively.
In the apparatus having the nonimaging areas, image formation cannot be performed until the photosensitive body 8 reaches a predetermined speed. For this reason, the developing unit 15 is arranged in the manner shown in FIG. 1. One of the nonimaging areas 8C and 8D always opposes the developing unit 15 when the apparatus is stopped. In this state, the toner on the sleeve 17 is in contact with the photosensitive body 8.